1. Field of the Invention
The present invention relates to a stopper for continuous casting used for regulating a flow rate of molten metal which is poured from a tundish into a mold in the continuous casting of metal, e.g., steel, a copper alloy, or an aluminum alloy or the like. In particular, the present invention relates to a tundish stopper for continuous casting which has a high gas sealing capability.
2. Related Art
In the continuous casting, molten metal is received in the tundish which is located above a mold of a continuous casting machine, and then, the molten metal is poured from the tundish through an immersion nozzle into the mold at a flow rate suitable for its casting condition. The tundish stopper for continuous casting regulates the flow rate to control a flow rate of the molten metal into the mold.
FIG. 1 shows a stopper rod 2 used within the tundish. As shown in the drawing, a gas supply route 21 is provided in the stopper rod so as to run through the stopper rod 2 along the center axis thereof, and gas is injected from an injection port 23 formed in the tip portion thereof. The above-mentioned gas comprises, for example, argon gas, which is supplied into an immersion nozzle (not shown) together with the molten metal of at least the fixed flow rate.
If the above-mentioned argon gas is not supplied, the flow of the molten steel generates a negative pressure within an inner bore of the immersion nozzle, and causes the air to enter the inner bore of the nozzle, resulting in oxidation of the molten steel. Consequently, so-called nozzle clogging occurs because of solidification of the molten metal or deposition of a nonmetallic inclusion. The stopper rod 2 includes a joining nut 6 provided in the center axis of its upper portion. A spindle is screwed to be engaged with this nut, thus the spindle is fixed.
FIG. 2 shows the stopper rod 2 having a spindle 4 fitted therein. A tip portion of the spindle 4 is fitted by a screw in the joining nut 6 embedded in the stopper rod 2. A flange 5 is disposed in the upper end portion of the stopper rod to realize firm fixation between the stopper rod 2 and the spindle 4. The flange 5 is fastened to the stopper rod by a fastening nut 12.
To make effective use of a gas introduced through a gas introduction route 40, an arrangement like that described below is proposed so as to prevent an upward leakage of the gas. More specifically, the gas supply route 21 includes a chamfered enlarged taper portion 22 and an enlarged cylindrical portion 24 in the upper side thereof. Corresponding to the above-mentioned features, the spindle includes a chamfered taper portion 42 and a flange portion 44. The enlarged taper portion 22 and the cylindrical portion 24 of the gas supply route are tightly adhered respectively to the taper portion 42 and the flange portion 44 of the spindle so as to secure hermetic sealing.
To secure hermetic sealing, mortar may be applied to a gap between the enlarged taper portion 22 and the cylindrical portion 24 (hereinafter referred to as a xe2x80x9cconcave portionxe2x80x9d) of the gas supply route 21 and the taper portion 42 and the flange portion 44 (hereinafter referred to as an xe2x80x9cenlarged portionxe2x80x9d) of the spindle. Thus, a space 26 is provided in the upper end part of the stopper rod, and a space portion 52 is provided in the flange 5 such that superfluous mortar can be released to these spaces.
However, for example when a depth of the molten steel in the tundish reaches 1 m, a static pressure becomes 0.7 atm. A flow velocity of the molten steel at the outlet of the stopper rod is high. In addition, since the stopper rod is made of an alumina graphite refractory, it is impossible to secure complete hermetic sealing, thus a part of atmospheric gas frequently enters the gas supply route 21 from the head portion of the stopper rod. Furthermore, to solve the problem of nozzle clogging caused by an inclusion or the like in the molten metal, the stopper may be moved up and down violently (so-called xe2x80x9cflappingxe2x80x9d). Such an up-and-down movement of the stopper generates play (in other wards, clearance) in the junction of the stopper rod and the spindle, further damaging the hermetic sealing.
Conventionally, a steel nut has been used as a joining nut formed integrally with the alumina graphite stopper rod. If a casting time is long, however, the carbons in the graphite may infiltrate the steel nut, and a melting point of the steel nut may be lowered, consequently breaking a screw thread. In addition, in the case of the steel nut formed integrally with the stopper rod, since a difference in coefficients of thermal expansion between a refractory material of the stopper and steel of the nut is large, when exposed to a high temperature, cracks occur in the vicinity of the boundary between the stopper and the nut because of the thermal spalling. Thus, play is generated between the stopper rod and the spindle.
As described above, it was impossible to secure hermetically sealed junction between the stopper rod and the spindle by the taper surfaces of both, and to assure smooth casting. In addition, in the case of the stopper having the integrally formed steel nut, for example in the continuous casting of the molten steel, the temperature of the nut reached about 700xc2x0 C., and play was generated between the nut and the spindle because of thermal expansion. As a result, hermetic sealing was lost between the stopper rod and the spindle. The present invention was made to solve the foregoing problems, and it is an object of the invention to provide a particular casting stopper having a high gas sealing capability.
In accordance with the present invention, there is provided a first embodiment of a stopper for continuous casting comprising:
a stopper rod including a gas supply route provided in a center portion thereof, and an injection port provided in a tip portion thereof for discharging to an outside a gas passed through the gas supply route; and
a spindle for actuating the stopper rod,
wherein said stopper rod includes a joining nut integrally formed in a position located at a specified distance from an upper end of the stopper rod and in coaxial relation to an axis of the gas supply route, an upper end surface of the stopper rod being formed so as to be smooth, and a pin hole being provided in the upper end surface so as to prevent from rotating;
said spindle includes a gas introduction route provided in a center portion thereof, a male screw provided in a tip portion thereof for being engaged with the joining nut, a male screw provided in an upper portion thereof for being engaged with a fastening nut for fixing the spindle to the stopper rod, and a flange tightly adhered to the upper end surface of the stopper rod and provided with a hole, into which a pin for preventing rotation is inserted; and
said spindle is fixed to the stopper rod by the joining nut, said flange is fixed to the upper end surface of the stopper rod by the fastening nut so as to be hermetically sealed, and said flange and said stopper rod are prevented from being rotated by the pin inserted into the pin hole.
A second embodiment of the invention comprises a stopper for continuous casting, wherein said flange of the spindle is formed integrally with the spindle.
A third embodiment of the invention comprises a stopper for continuous casting, wherein said flange of the spindle is formed by being welded to the spindle.
A forth embodiment of the invention comprises a stopper for continuous casting, wherein said flange of the spindle and said upper end surface of the stopper rod are fixed to each other in a hermetically sealed manner by applying mortar, alternatively holding sealant therebetween.
A fifth embodiment of the invention comprises a stopper for continuous casting, wherein said joining nut is made of structural fine ceramics having a bending strength of at least 100 MPa at a room temperature.
A sixth embodiment of the invention comprises a stopper for continuous casting, wherein said structural fine ceramics comprises any one selected from alumina, mullite, silicon carbide, silicon nitride, sialon and zirconia, or a composite material thereof.